1. Field of the Invention
The present invention relates to zooming optical systems, imaging optical devices, and digital devices. For example, the present invention relates to compact zooming optical systems suitable for interchangeable-lens digital cameras that capture an image of a subject by use of an image sensing device, imaging optical devices that output in the form of an electrical signal the image of the subject captured by the zooming optical system and the image sensing device, and digital devices equipped with an image input function, such as digital cameras, that incorporate such imaging optical devices.
2. Description of Related Art
In conventional zooming optical systems used as interchangeable lenses for single-lens-reflex cameras, the mainstream zoom types have been a positive-negative-positive-positive type as disclosed in Patent Document 1 listed below and a positive-negative-positive-negative-positive type as disclosed in Patent Documents 2 and 3 listed below. On the other hand, in recent years, mirrorless interchangeable-lens cameras, which no longer have a swing-up mirror as is provided in single-lens-reflex cameras, have been gaining acceptance among users for their compactness, and their market has been growing. In those mirrorless cameras, it is aimed at to effectively exploit the space that would have been reserved for a mirror, in order to achieve further improved compactness in the state with a lens attached, that is, to provide an optical solution that exploits the shorter lens back in order to make the optical system compact.    Patent Document 1: Japanese Patent Application Publication No. 2009-271471    Patent Document 2: Japanese Patent Application Publication No 2011-221422    Patent Document 3: Japanese Patent Application Publication No. 2011-237588
In this respect, the lens systems disclosed in Patent Documents 1 and 2 mentioned above, which provide optical solutions on the assumption that a mirror space has to be reserved, cannot be said to be satisfactorily compact. On the other hand, Example 1 disclosed in Patent Document 3 does not assume reservation of a mirror space, and realizes a compact optical system as a whole with a shorter lens back than ever. However, the movement amounts of individual groups are large, and thus it still cannot be said that satisfactory compactness is achieved.